Gas filter

ABSTRACT

An ostomy filter for colostomy and ileostomy bags is gas-permeable to allow venting, and liquid-impermeable to prevent leakage. It includes a hydrogen sulphide adsorbent to remove smells. The filter comprises 9 to 50% by weight of particles of unsintered granular polytetrafluoroethylene (PTFE), 30 to 82% by weight of particles of comminuted sintered expanded porous PTFE, and 9 to 30% by weight of particulate water-insoluble H 2  S adsorbent. The particles are fused together by baking at elevated temperature to form a gas-permeable liquid-impermeable network. The comminuted sintered expanded porous PTFE is preferably formed by comminuting uniaxially expanded PTFE fibre.

FIELD OF THE INVENTION

The present invention relates to an ostomy filter which allows gas to bevented from colostomy and ileostomy bags, whilst at the same timefiltering out any unpleasant smell, particularly hydrogen sulphide.

BACKGROUND

After undergoing a colostomy or ileostomy operation, the patient isfitted with a disposable bag which receives the waste contents of theintestine. The bag is disposable and is emptied or replaced at regularintervals. To avoid the bag becoming over inflated by virtue of gasgenerated by the fermenting food matter, it is desirable to provide avent. However, the vent requires to be filtered in order to preventembarrassing smells being released.

It is currently conventional practise in colostomy bags to provide anactivated charcoal filter in the form of a disc as the vent. For examplethe disc may have a diameter of 23.5 mm and be formed of activatedcarbon contained within a foamed polyurethane material, havingpolypropylene net facings on either or both sides to improve itsstrength and other mechanical properties. However, this conventionalfilter material has the significant disadvantage of beingliquid-permeable, so that if the patient sits or lies down there is asignificant danger of leakage of liquid through the filter onto thepatient's skin or his clothing. This is particularly the case withileostomy bags, where the bag is connected into the early part of theintestine where the food matter is largely undigested and stillfermenting, and has a high liquid content. In fact conventionalileostomy bags are not provided with vents, and the filters used forcolostomy bags would be quickly saturated with liquid if used forileostomy.

A porous polytetrafluoroethylene structure containing active carbon isdisclosed in our British patent application GB2242431. However, thematerial disclosed is insufficiently gas permeable.

It is an object of the present invention to provide an ostomy filterwhich mitigates these problems.

Patent specification GB 1399213 discloses a gas-permeable liquidimpermeable membrane which includes finely divided high surface areacarbon for the production of hydrogen peroxide.

SUMMARY OF THE INVENTION

The invention provides a gas permeable and liquid impermeable ostomyfilter for colostomy and ileostomy bags which comprises a mixture of;

(i) particles of unsintered granular-type polytetrafluoroethylene(PTFE);

(ii) particles of comminuted sintered expanded porouspolytetrafluoroethylene (PTFE); and

(iii) a particulate water-insoluble hydrogen sulphide adsorbent; theparticles being fused together such as to form a gas permeable liquidimpermeable network of interconnected particles.

PREFERRED EMBODIMENTS

The permeable nature of the polytetrafluoroethylene (PTFE) structureresults from the particulate nature of the PTFE used to form it. Thebaking conditions are chosen such that the PTFE particles becomepartially fused or sintered into an integral permeable network duringbaking.

A feature of the present invention is that granular type PTFE isemployed as a component. As is well known, PTFE can exist in two quitedifferent forms viz; the granular form produced by precipitationpolymerisation techniques, and the powdered form produced by dispersionpolymerisation processes (i.e. fine-powder PTFE). These two types ofPTFE have quite different properties and have to be consideredseparately.

Unsintered granular PTFE generally has a crystallinity in excess of 95%as determined by differential scanning calorimetry or by infra-redtechniques. The degree of crystallinity is reduced by sintering (i.e.baking), and the finished structure of the present invention willtypically have a crystallinity of less than 95%, often less than 80%. Anunsintered granular PTFE is available from DuPont as Grade 7A.

The average particle size of the granular-type PTFE is usually in therange 1-500, particularly 1-200, especially 1-100 microns. Depending onthe desired porosity of the structure, the particles may havesubstantially the same particle size, or a range of varying particlesizes may be included which pack to influence the porosity of thestructure. The granular PTFE may be milled or unmilled.

The comminuted porous expanded polytetrafluoroethylene (PTFE) materialcomprises irregularly shaped particles of porous expanded PTFE. Thestarting material from which the comminuted expanded PTFE particles areproduced may be any form of sintered porous expanded PTFE, such as film,sheet, rod, fibre or tube. It is preferably produced by comminuting(e.g. by chopping or grinding) expanded porous PTFE material (producedas described in U.S. Pat. No. 3,953,566) to a particle size of 5 to 200microns, particularly 40-100 microns. Generally speaking, the lowerlimit of particle size is 5 microns, preferably 20 microns and morepreferably 40 microns; whilst the upper limit is 500 microns, preferably200 microns and more preferably 100 microns. The structure of porousexpanded PTFE is well known, being characterized by nodes interconnectedby fibrils as described in U.S. Pat. No. 3,953,566 to form a waterproof,but water vapor-permeable porous material. The expanded PTFE is sinteredto give dimensional stability, and to mitigate shrinkage during bakingof the mixture to form the filter material. The particles of sinteredporous PTFE may be produced by shredding or chopping sintered porousexpanded PTFE to reduce it to small pieces, and then grinding theseporous expanded PTFE pieces in water between closely spaced grindingsurfaces to obtain sheared and ground particles of porous expanded PTFE,separating the sheared and ground particles from the water and dryingthe separated particles. Any suitable apparatus for grinding orcomminuting tough polymeric or elastomeric materials may be used forproducing the porous expanded PTFE particles, such as the apparatusdisclosed in U.S. Pat. Nos. 4,614,310 and 4,841,623.

In order to deodorise the gas vented from the ostomy bag, hydrogensulphide is filtered out by adsorption. The hydrogen sulphide adsorbentpreferably includes a high surface area support material impregnatedwith a water-insoluble H₂ S-adsorbent substance. The support materialmay be any suitable material known in the art, such as activated carbon,diatomaceous earth etc. Activated carbon is commercially available andis formed, for example, by heating carbon with steam to 800°-900° C. Inorder to avoid removal or deactivation by water, the H₂ S-adsorbent iswater-insoluble, and typically comprises a water-insoluble copper oxide.

A preferred material is activated carbon impregnated with copper oxide.This may be produced by soaking the activated carbon in cupric nitratesolution to produce cupric nitrate, and then drying and igniting togenerate black copper oxide attached to the activated carbon. The copperoxide is insoluble in water or ethanol. Hydrogen sulphide is adsorbed byreaction to produce copper sulphide.

In order to provide good H₂ S adsorption, the filter preferablycomprises 10 to 30% by weight of impregnated high surface areaadsorptive material. Typically, the H₂ S-adsorbent constitutes 2 to 10%of the weight of the support material.

Particularly preferred embodiments of the invention comprise thecomponents (i)-(iii) as defined above in the following weightpercentages;

(i) 9-50%, preferably 10-40% (or 30-50%);

(ii) 30-82%, preferably 40-80% (or 30-50%); and

(iii) 9-30%, preferably 10-20% (or 10-30%).

The filter material is generally formed by applying to a substrate asuspension comprising the mixture of PTFE particles and impregnatedsupport material, allowing to dry and baking at a temperature usually inthe range 335° to 350° C. for 0.5 to 3 hours. However, the conditionsmay be varied when the process is conducted as a continuous operation,when reduced heating times of 1 to 30 mins may be used. The thicknessmay be created in a single step or may be built up by applying a numberof layers and allowing each to dry between applications, prior to bakingthe layered composite. Generally, the process is conducted atsubstantially atmospheric pressure. Spraying ensures that no air pocketsare left between the substrate and the coating. However, other knownapplication techniques, such as a doctor blade can be used. Thesuspension is usually an aqueous suspension comprising appropriatesurfactants, thickening agents and/or suspending agents. The temperaturemay be progressively raised over a few hours e.g. 1 to 10 hours in orderto remove such additives, prior to baking.

The structure of the present invention generally has a specific gravityof 0.5 to 1.0, usually 0.6 to 0.9. The specific gravity tends to go downas the proportion of comminuted PTFE increases. In comparison, solidPTFE typically has a specific gravity of 2.16. It is important to getgood gas permeability commensurate with acceptable physical strength.

The gas permeable PTFE filter is generally hydrophobic in nature andtypically has a water entry pressure (measured as described in GB2242431) in excess of 1.5, preferably 2.0 lbs per square inch. It istherefore liquid impermeable under normal conditions of usage.

The thickness of the filter will vary depending on the desired gaspermeability and degree of adsorption, but is usually in the region 0.8to 6 mm, preferably 1 to 5 mm, and especially 1.5 to 2.5 mm.

The filter preferably allows a gas flow rate in excess of 1 liter perhour, preferably at least 5 liters, more preferably at least 10 litersper hour. Typical gas flow rates from a patient are approximately 2.5liters per day, though peak flow rates during a day may exceed this. Thefilter is preferably capable of maintaining a level of H₂ S in thefiltered gas below one part per million for at least 20, preferably 40minutes. In normal use on a patient this means that the filter wouldrequire changing every day or two. Principal gas flows occur after eachmeal as the food is digested. The filter is usually in the shape of adisc and may be arranged for flow of gas in a direction perpendicular tothe plane of the disc. Alternatively for a longer flow path, it may bearranged to flow radially from a central entry point to the periphery ofthe disc (or vice versa). This may be achieved by skinning the surfaceof the filter disc during production and providing a central pinholeentry, or by masking the corresponding areas.

The ostomy filter of the present invention has a high water entrypressure which makes the filter impervious to liquid water under normalconditions of operation, whilst having good gas permeability and H₂ Sremoval capacity. By including impregnants for different gases in thesupport material, the filter can be adapted to remove gases other thanhydrogen sulphide.

The invention also relates to an ostomy bag including the filter, eitheras a removable replaceable filter element or integral with the bag (e.g.heat welded to the bag).

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Embodiments of the invention will now be described by way of exampleonly.

PREPARATION OF COMMINUTED EXPANDED POROUS POLYTETRAFLUOROETHYLENEPARTICLES.

Comminuted expanded porous PTFE was produced from sintered porousexpanded PTFE material obtained from W. L. Gore & Associates, Inc. insheet, rod, fibre or tube form. The porous expanded PTFE material wascut into pieces of about 6 mm (quarter inch) largest dimension in arotary cutting mill. The cut material was mixed with water to form aslurry, and the slurry fed between closely spaced grinding surfaces of agrinding mill, such as that disclosed in U.S. Pat. No. 4,841,623, tocrush and shear the pieces of porous expanded PTFE into particles. Theground slurry is then filtered or centrifuged to separate the porousexpanded PTFE particles from the water, and the separated finely groundparticles were oven dried at from 125° C.-150° C.

The product was a comminuted porous expanded PTFE material comprisingfinely ground particles of irregular shape.

The comminuted particles preferably will have a mean particle sizebetween 20 and 500 microns, preferably between 40 and 200 microns.Particle size was determined as follows: using a magnetic stirrer andultrasonic agitation, 2.5 grams of milled PTFE powder were dispersed in60 ml isopropyl alcohol. (Ultrasonic Probe Model W-385, manufactured byHeat Systems-Ultrasonics, Inc.). Aliquots of 4-6 ml of the dispersedparticles were added to approximately 250 ml of circulating isopropylalcohol in a Leeds & Northup Microtrac FRA Particle Size Analyzer ofanalysis. Each analysis consisted of three 30-second runs at a samplecirculation rate of 2 liters/minute during which light scattering by thedispersed particles is automatically measured and the particle sizedistribution automatically calculated from the measurements.

The particles will preferably have an average surface area of between 1and 3 m² /gm, as determined by specific surface area measured by a Leedsand Northrup surface area analyzer. The surface area analyzer uses theBET (1) method to calculate surface area. In this sample analysis, thedesorption isotherm of a single point analysis was used to calculate thesurface area.

EXAMPLE 1

The general procedure outlined in British Patent Publication GB 2242431Awas followed. Teflon grade 7A resin (DuPont Speciality PolymersDivision, Wilmington, U.S.A., average specific gravity 2.16 and particlesize 35 microns) was mixed with the comminuted PTFE (particle size 50 to100 microns), and activated carbon grade 209M (Sutcliffe Speakman) inthe following percentages by weight:

Solids:

40% PTFE grade 7A

40% comminuted PTFE

20% activated carbon 209M

The solids were then blended in a Waring blender for 5 mins with liquidof the following composition by weight, in a ratio of 1 kg solids to 2 lof liquid, to produce a suspension.

Liquid:

43% water

43% carboxymethyl cellulose

6% Zonyl FSN100 surfactant

8% Pluronic L121 surfactant

Pluronics are a class of polyethylenepolypropylene glycol non-ionicsurfactants.

The suspension was then sprayed as a coating onto a metal tray (asdescribed in GB 2242431A) as a single layer. The thickness of thesprayed layer was about 20% greater than the desired thickness in thefinished sample, to allow for shrinkage during baking. The coating wasthen dried in an oven at 115° C. for 30 mins. The temperature was raisedto 280° C; then progressively to 333°-348° C. and baked at thattemperature for 60 mins.

The specific gravity of the baked coating was 0.78. The water entrypressure was approximately 2 lbs per square inch.

EXAMPLE 2

Samples 1,2 and 3 as produced in Example 1 of varying thickness were cutinto discs 23 mm in diameter (4.16 cm²) and tested for gasdeodorisation. The test gas comprised 80% nitrogen, 20% methane and 25ppm H₂ S, and the flow rate was 0.5 l/min. The test apparatus compriseda sample holder, a test gas inlet, an H₂ S detector and read-out unit.

During the test the gas is allowed to flow to the centre of the filtersample, which is clamped in position in the holder. The gas flow throughthe sample may be axial or radial (for an increased path). The H₂ Sdetector attached to the other side of the sample monitors theefficiency of the filter in removing the hydrogen sulphide. Once thisefficiency begins to fall below 100% the detector reading is notedagainst time. The results are expressed as the time the electrochemicaldetector remains at zero (and moves to 0.1 ppm) as well as the timetaken to reach 1.0 ppm.

The results are shown in Table 1.

                  TABLE 1    ______________________________________          Thick-   Gas*       Time to detect                                        Gas          ness     deodorisation                              [H.sub.2 S] 1.0 ppm                                        permeability    Sample          mm       liters     (minutes) cc/minute**    ______________________________________    1     2.7      9          18        125.0    2     3.3      15         30        30.0    3     4.0      52.5       105       25.0    ______________________________________     *target value = 9     **pressure of 4 cm waterhead

EXAMPLE 3

The general procedure of Example 1 was repeated with two furthercompositions as follows:

                  TABLE 2    ______________________________________    Sample                4      5    ______________________________________    PTFE grade 7A         10%    20%    Comminuted PTFE       80%    60%    Activated carbon 209M 10%    20%    Gas permeability cc/min/cm.sup.2                          4000   1000    ______________________________________

The comminuted PTFE was formed from Rastex (trademark) fibre expanded at80:1 and of particle size 50 to 100 microns.

The drying regime employed was as follows. The wet sprayed coating wasoven dried at 90° C. overnight. The temperature was then raised to 115°C. Then the temperature was raised progressively from 115° C. to 340° C.over 7 hours. Finally, the coating was baked at 340° C. for 30 mins.

The specific gravity was about 0.6-0.8 g/cc.

The coating was cut into discs about 23 mm diameter and 2 mm thick, andthe gas permeability measured using a Gurley Densometer according tostandard protocol. The Gurley number is defined as the time in secondsfor 100 cc of air to pass through one square inch of filter at 4.88inches of water pressure differential. The Gurley number is thenconverted into a gas flow rate in cc/min/cm². The gas permeabilities aregiven in Table 2.

We claim:
 1. A gas permeable and liquid impermeable hydrogen sulfidefilter which comprises a mixture of:(I) particles of unsintered granularpolytetrafluoroethylene (PTFE); (ii) particles of comminuted sinteredexpanded porous polytetrafluoroethylene (PTFE); and (iii) a particulatewater-insoluble hydrogen sulphide adsorbent; the particles defined in(I) and (ii) above being fused together such as to form a gas permeableliquid-impermeable network of interconnected particles.
 2. A filteraccording to claim 1 which comprises:9 to 50% by weight of component (I)of claim 1; 30 to 82% by weight of component (ii) of claim 1; and 9 to30% by weight of component (iii) of claim
 1. 3. A filter according toclaim 1 which comprises:10% to 40% by weight of component (i) of claim1; 40 to 80% by weight of component (ii) of claim 1; and 10 to 20% byweight of component (iii) of claim
 1. 4. A filter according to claim 1which comprises:30 to 50% by weight of component (I) of claim 1; 30 to50% by weight of component (ii) of claim 1; and 10 to 30% by weight ofcomponent (iii) of claim
 1. 5. A filter according to claim 1 wherein theparticles of comminuted porous PTFE have a particle size in the range 40to 200 microns.
 6. A filter according to claim 1 wherein the expandedporous PTFE, from which said comminuted PTFE particles are derived, ismaterial which has been uniaxially expanded to form fibre prior tocomminution.
 7. A filter according to claim 1 wherein the particles ofcomponent (ii) are derived from expanded porous PTFE that has beenbiaxially expanded to form sheet prior to comminution.
 8. A filteraccording to claim 1 wherein the hydrogen sulphide adsorbent isactivated carbon impregnated with copper oxide.
 9. A filter according toclaim 1 having a specific gravity of 0.5 to 1.0 g/cc.
 10. A filteraccording to claim 1 in the form of a disc of thickness 1.0 to 5.0 mm.11. A filter according to claim 1 having a gas permeability of at least500 cc/min/cm².
 12. A gas permeable and liquid-impermeable filter whichcomprises a mixture of:(I) particles of unsintered granularpolytetrafluoroethylene (PTFE); (ii) particles of comminuted sinteredexpanded porous polytetrafluoroethylene (PTFE); and (iii) a particulatewater-insoluble adsorbent comprising activated carbon impregnated withcopper oxide;the particles defined in (I) and (ii) above being fusedtogether such as to form a gas permeable liquid-impermeable network ofinterconnected particles.
 13. A gas-permeable liquid-impermeablematerial which comprises a mixture of the following components:(I)particles of unsintered granular polytetrafluoroethylene; and (ii)particles of comminuted sintered expanded porouspolytetrafluoroethylene;the particles defined in (I) and (ii) abovebeing fused together such as to form a gas permeable liquid-impermeablenetwork of interconnected particles.
 14. A process for the production ofa gas permeable and liquid-impermeable hydrogen sulphide filter materialwhich comprises:(A) forming a mixture in liquid suspension of:(i)particles of unsintered granular polytetrafluoroethylene (PTFE); (ii)particles of comminuted sintered expanded porous polytetrafluoroethylene(PTFE); and (iii) a particulate water-insoluble hydrogen sulphideadsorbent; (B) applying the liquid suspension to a substrate andallowing to dry; and (C) baking so as to fuse the particles defined in(i) and (ii) above together such as to form a gas permeableliquid-impermeable network of interconnected particles.
 15. A processaccording to claim 14 wherein the temperature of the dried suspension isgradually raised to baking temperature over 1 to 10 hours.
 16. A processaccording to claim 14 or 15 wherein baking occurs at 335° to 350° C. for0.5 to 3 hours.